The question above is from Yahoo!Answers. And I believe this is a common issue may due to BOOTMGR is missing on start up happen to lots of computer users. So, it's necessary to talk about this problem today. When such thing happen to us, we have two choices.
Choice 1: Use a Windows startup disk
One of the first things to deal with windows won't start you should reach for when trouble shooting a Windows XP boot problem is a Windows startup disk. This floppydisk can come in handy if the problem is being caused when either the start up record for the active partition or the files that the operating system uses tostart Windows have become corrupted.
To create a Windows start up disk, insert a floppy disk intothe drive of a similarly configured, working Windows XP system, launch MyComputer, right-click the floppy disk icon, and select the Format command fromthe context menu. When you see the Format dialog box, leave all the defaultsettings as they are and click the Start button. Once the format operation is complete, close the Format dialog box to return to My Computer, double-clickthe drive C icon to access the root directory, and copy the following three files to the floppy disk:
* BOOTMGR is missing
After you create the Windows startup disk, insert it intothe floppy drive on the afflicted system and press [Ctrl][Alt][Delete] to reboot the computer. When you boot from the Windows startup disk, the computerwill bypass the active partition and boot files on the hard disk and attempt tostart Windows XP normally.
Choice 2: Windows Data Recovery Before Re-install System.
If choice 1 can't work, I'm afriad you have to re-install system. However, many people would not want to re-install system because they do not want to lose the important files in Disk C. As I know, a software called Tenorshare Data Recovery can help to backup the important files when windows won't start up .It need burn into CD/DVD,or USB flash first, and then set the Bios to enter the software recovery windows, then you can choose the data you want, and save in other disk except the disk C. I have tested it, very effective indeed.You can download it from http://www.any-data-recovery.com/product/datarecoveryprofessional.htmWarning
Computer won't boot due to computer poisoning in many cases, therefore, regular computer antivirus is necessary.